The present invention relates to a heavy duty pneumatic tire having an improved belt structure.
The heavy duty pneumatic tires for heavy vehicles such as trucks, buses and the like are usually provided in the tread portion with a stiff belt, which belt includes a plurality of steel cord breaker plies. The breaker plies are made of conventional steel cords which are well known in the art such as: Regular cord (a cord in which the direction of lay in the strands is opposite to the direction of lay in closing the cord); Lang""s lay cord (a cord in which the direction of lay in the strands is the same as the direction of lay in closing the cord); and Compact cord (CC) (a wrapped cord in which the filaments mainly have linear contact with each other).
Recently, a steel cord in which some (a small number) of the filaments are replaced by waving filament(s) has been proposed, example, in U.S. Pat. No. 5,351,470. This replacement of unwaving filaments with waving filaments aims to improve the penetration of rubber into the cord.
Such steel cord is certainly improved in the penetration of rubber. However, due to the difference between the length of the unwaving filaments and the length of the waving filaments, a tension imbalance occurs and the cord is liable to be broken under the extreme conditions that a heavy duty pneumatic tires is often exposed.
It is a primary object of the present invention to provide a heavy duty pneumatic tire having an improved belt in which, by improving the resistance to breakage of the steel cords used in the belt, the belt weight can be decreased causing a deteriorating in the performances.
According to one aspect of the present invention, a heavy duty pneumatic tire comprises
a tread portion,
a pair of axially spaced bead portions,
a pair of sidewall portions extending between the tread edges and the bead portions,
a toroidal carcass extending between the bead portions, and
a belt disposed radially outside the carcass and radially inside the tread portion, the belt comprising at least one ply of at least one high-elongation steel cord.
Each of the at least one high-elongation steel cords is formed by twisting at least three waving steel filaments together to form a core and a sheath surrounding the core.
The least three waving filaments change place repeatedly along the longitudinal direction of the cord such that one of the filaments forms the core at a portion thereof. In a different portion of the cord, the same filament forms a part of the sheath, thereby defining a plurality of first parts with the core, and a plurality of second parts without the core which are disposed alternately in the longitudinal direction of the cord. The elongation at break of each of said at least one high-elongation steel cord is in the range of from 4 to 10%.
According to another aspect of the present invention, a tire comprises a reinforcing layer made of at least one high-elongation steel cord.
Each of the at least one high-elongation steel cord is formed by twisting at least three steel filaments together to form a core and a sheath surrounding the core.
The at least three filaments change place repeatedly along the longitudinal direction of the cord such that one of the filaments forms the core at a portion thereof. In a different portion of the cord, the same filament forms a part of the sheath, thereby defining a plurality of first parts with the core and plurality of second parts without the core, which are disposed alternately in the longitudinal direction of the cord.